1. Field of the Invention
The present invention relates to stereo sound image enhancement, and more particularly concerns methods and apparatus for providing surround sound having enhanced directivity and enhanced ambience.
2. Description of Related Art
Surround sound systems endeavor to provide more realistic sound imaging and to have the listener experience the sound as if he were positioned within the area of action depicted by the sound. Such surround sound systems are, at present, more commonly employed in large chambers, such as movie theaters and the like, but are finding ever increasing application in home stereo systems.
Typically, a surround system employs a set of speakers at the front of the theater and a set of speakers at the rear of the theater. Additional speakers along the theater sides may be employed. Many different techniques have been suggested and are presently used for processing sound signals, whether stereo or monaural, and feeding these to front and rear speakers of the theater speaker system.
Signals fed to front and rear speakers must be processed differently in order to keep the listener's attention focused at the front of the theater where the visual display is located. In a movie theater, for example, the action is seen at the front and sound is heard from the front. At the same time, surround sound is provided from the rear or sides to augment ambience, width and scope of the sound image while attempting to maintain primary attention of the listener directed to the front of the theater.
Prior systems arranged to provide surround sound include DOLBY surround, DOLBY pro-logic and FOSGATE systems. In the DOLBY system, for example, four uniquely prepared surround input channels, including left, right, center and surround channels, are matrixed down to two channels by a stereo matrix encoder. These two channels are provided as a sound source, either broadcast or fixed on a sound recording medium, such as a record, tape, compact disc or the like. Left and right front channels are not altered, but a center channel, representing the sum of left and right channels, is used at a level of 3 db down, for obtaining equal loudness considerations, and is added to both right and left side information. A surround channel, also reduced by 3 db, is shifted in phase plus or minus 90.degree. for left and right channel information.
The two output signals, e.g. left and right channels, can be fed from the sound source to a variety of speaker systems. They can be fed to a monaural system, a conventional stereo speaker system having left and right speakers, or to a specific surround speaker system particularly arranged for maximum utilization of the pair of matrixed DOLBY surround signals. However, for use in such a surround speaker system, a decoding or de-matrixing circuit is required to process the two output signals for transmission to the front and rear speakers of the theater or the like.
Such surround systems are considered by many to provide significant improvement in large theater sound, but, nevertheless, exhibit a significant weakness in their attempted realistic sound production. This weakness is due to the fact that the sound often appears to emanate from a point source in such systems. Sound heard by persons seated at one location in the theater may be quite different from sound received by a person seated at a different location. Generally in such systems a seat in the center toward the front provides maximum desirable effect of the surround sound.
The point source problem is caused, in part, by the fact that sound coming from one speaker is louder when it reaches the listener if the listener's position is closer to such one speaker. Thus, for example, a person sitting at the rear of the theater, close to one of the rear speakers, may find that sound coming from the closest rear speaker tends to dominate sound that he receives from speakers at the front. This tends to focus the listener's attention at a point at the rear of the theater closer to his seat rather than at the front of the theater where the visual activities occur. This is undesirable.
This point source problem is alleviated to some extent by taking advantage of the Haas effect, which recognizes that one sound reaching a listener prior to a second sound tends to dominate the second sound, and that relative intensities of the two sounds at the listener may be somewhat compensated by interjecting relative time delay. Accordingly, sound sent to speakers in the rear of the theater is commonly delayed to some extent in attempts to maintain focus of the listener's attention at the front of the theater. Thus front emanating sounds may reach the listener in the rear before he hears rear emanating sounds, and therefore at least partially compensate for his greater proximity to the rear speakers. Nevertheless, despite such attempts, the point source effect, which tends to localize sound sources at individual speaker locations, still predominates. A related problem is the fact that the sound image provided to the listener in the theater varies with seat location.
Accordingly, it is an object of the present invention to provide a surround system that avoids or minimizes above mentioned problems.